sftp: Prompt for password, don't terminate on SIGINT

This is a follow-up on fb9729fdb9, which
runs the `ssh` in its own process group and selects that process group
as the foreground group. After the sftp connection is established,
restic switches back to the previous foreground process group.

This allows `ssh` to prompt for the password, but it won't receive
the interrupt signal (SIGINT, ^C) later on, because it is not in the
foreground process group any more, allowing a clean tear down.

internal/backend/sftp/sftp.go

@@ -69,8 +69,8 @@ func startClient(preExec, postExec func(), program string, args ...string) (*SFT
 		preExec()
 	}
 
-	// start the process
-	if err := cmd.Start(); err != nil {
+	bg, err := startForeground(cmd)
+	if err != nil {
 		return nil, errors.Wrap(err, "cmd.Start")
 	}
 
@@ -92,6 +92,11 @@ func startClient(preExec, postExec func(), program string, args ...string) (*SFT
 		return nil, errors.Errorf("unable to start the sftp session, error: %v", err)
 	}
 
+	err = bg()
+	if err != nil {
+		return nil, errors.Wrap(err, "bg")
+	}
+
 	return &SFTP{c: client, cmd: cmd, result: ch}, nil
 }